22 / 55
20
•
the resistance to pollutants, especially in the strictly urban environments
26;
•
resistance to pathogens and plant parasit
e 27;
•
toxicity (leaves, fruits, sap), aspect to consider in function of the security of citizens (for
example, if the new area will be very fruited by children) and in relation to animal
biodiversity;
•
the frequency of maintenance (for example species with particular needs for water resources),
which affects the costs of management;
•
longevity, especially in urban areas where the stressors may increase the rate of mortality of
individuals.
Regardless of the function that the new forested area will must have, primary aspects to consider
are, however, the size and shape of the canopy. In reference to the size in the Green Regulations are
generally shown heights of plants at maturity for size classes. For the city of Rome, in urban area, the
size classes are so defined in the Regulations of Green and Urban Landscape
28:
Class size
Height of plants to maturity
1
st
size
> 18 mt
2
nd
size
12 – 18 mt
3
rd
size
< 12 mt
For example the Green Regulations of Roma Capitale
29mentions the following species :
•
1
st
size: pine (
Pinus pinea
) black poplar (
Populus nigra
), Atlas cedar (
Cedrus atlantica
), etc.;
•
2
nd
size: holm oak (
Quercus ilex
), Mediterranean cypress
(Cupressus sempervirens
) , elm
(
Ulmus minor
), etc.;
•
3
rd
size: field maple (
Acer campestre
), Judas tree (
Cercis siliquastrum
), privet (
Ligustrum
lucidum
), etc.
In general in the planning should be given preference to a mixture of species of various
dimensions: this allows a greater stability and resistance of the plant community that is to realize. In
addition it will be more aesthetically pleasing and favorable to establishment of more varied habitats
for the fauna.
A mixture of species may increase the multifunctionality of forestry intervention, because different
species can perform with different effectiveness various functions (mitigation of pollution, CO
2
storage, organication of NO
x,
encouragement of biodiversity, etc.) .
Even in the case of tree-covered road, it can be useful to design multi-species rows, as in general
are often carried monospecific, especially in urban areas (both for aesthetic that maintenance).
For a rational use of the species is also fundamental an adequate knowledge of local soil type and
of its hydrological conditions at the end of the installation. In particular, it is important to give priority
importance to the acidity, moisture (especially if seasonal or permanent) and porosity.
Nor the component represented by micoflora symbiont and saprophyte had be neglected, because it
improves the condition of the soil and the health of the tree species and is supported by plurispecificity
of the intervention.
In the Mediterranean context it is appropriate to consider some limiting factors such as the presence
of a summer with water stress, and the presence of a dormant period shorter than that of mountain
regions
(AA.VV., 2010)
30.
Therefore it is necessary to have more tricks in the choice of species and
their retrieval, realizing appropriate analysis of flora and vegetation in relation to existing ecological
gradients in the areas of plantation (see 1.5).
In the redevelopment of urban areas assumes great importance also planting fruit trees belonging to
local rare or endangered cultivars or clones of patriarchs arboreal (a good example was made in Rome
26
Some species are particularly sensitive to the increased level of air pollution, such as the horse chestnut (
Aesculus hippocastanum
) and
some species of oak (Lassini et al., 1998.
Forestazione urbana per la Lombardia
. Regione Lombardia e Azienda Regionale delle Foreste).
27
Such species, very used in the past, especially in tree-lined rows, are the plane tree (
Platanus
spp.) and elm (
Ulmus
spp.), though both are
subject to lethal infections. For example in the square Prato della Valle in Padua the plane trees, infected by canker stain of plane (the last
one was shot down in 2011), have been replaced from curly maple trees. The elm, instead, can undergo defoliation by the beetle
Galerucella
elm, whose repeated attacks can weaken the plants predisposing them to other infestations.
28
The size classes may vary between the various cities according to bioclimatic conditions (e.g. in Turin Class 1 includes trees up to 16
meters).
29
Regulations of Green and Urban Landscape of Rome is on the definition, but is not yet approved.
30
AA.VV., 2010.
Analisi e progettazione botanica per gli interventi di mitigazione degli impatti delle infrastrutture lineari
. ISPRA Manuali
e linee guida 65.3/2010